Two sided slow wave transmission circuit having a selective mode coupler



Dec. 13, 1966 c ENDERBY ETAL 3,292,112

TWO SIDED SLOW WAVE TRANSMISSION CIRCUIT HAVING A SELECTIVE MODE COUPLERFiled April 28, 1964 2 Sheets-Sheet 1 INVENTORS CHARLES E. ENDERBYRIgARD M. WHlTE ATTORNFV 1966 c. E. ENDERBY ETAL 3,

TWO SIDED SLOW WAVE TRANSMISSION CIRCUIT HAVING A SELECTIVE MODE COUPLERFiled April 28, 1964 2 Sheets-Sheet 2 INVENTORS CHARLES E. ENDERBY iglpHARD M. WHITE ATTORNEY United States Patent 3,292,112 TWO SIDED SLOWWAVE TRANSMISSION CIR- CUIT HAVING A SELECTIVE MODE COUPLER Charles E.Enderby, Palo Alto, and Richard M. Whlte,

Berkeley, Calif., assignors to General Electric Company, a corporationof New York Filed Apr. 28, 1964, Ser. No. 363,201 4 Claims. (Cl. 333-6)This invention relates to a structure for coupling electromagnetic wavesin a particular mode in a waveguide or the like to and/ or from amulti-mode wave transmission or propagation structure.

The selective coupler of the invention has a particular utility as theinput and/or output coupling means -for the interaction circuit of atraveling-wave tube or the like.

In the well-known traveling-wave tube a beam of electrons is projectedby an electron gun closely past an interaction circuit along which anelectromagnetic wave is propagated, the beam eventually beingintercepted by a collector electrode.

Numerous types of interaction circuits have been proposed for use insuch tubes. For example, the wellknown helix slow-wave circuit has beenwidely used. However, the helix circuit is not found practical forhigher frequencies and higher power because of small physical size andlimited power dissipation. Thus as the trend toward higher power and theuse of millimeter and subrnillimeter waves continues, other types ofslow-wave circuits are found more practical. Among these are thefollowing: the ladder circuits as discussed, for example, by R. M.White, C. K. Birdsall and R. W. Grow in'Multiple Ladder Circuits forMillimeter Wavelength Traveling-Wave Tubes, pp. 367-402, Proceedings ofthe Symposium on Millimeter Waves, Polytechnic Institute of Brooklyn,New York, 1959; the ring-bar circuits as discussed, for example, by C.K. Birdsall and T. E. Everhart in Modified Contra-Wound Helix Circuitsfor High- Power Traveling-Wave Tubes, pp. l90-204, Trans. IRE PGED, vol.3, October, 1956; and the ring-plane circuits as discussed for exampleby R. M. White et al. in Additional Information on An InteractionCircuit for Traveling-Wave Tubes, pp. 1686 and 1687, Proc. IRE, vol. 49,November 196-1.

In the use of some versions of these circuits, the thick ladder circuit,the double ladder circuit, the ring-plane circuit as examples, a seriousproblem arises from the fact that the circuit propagates two modes ofnearly equal phase velocities. It is therefore difficult to prop-agatethe electromagnetic wave in the desired mode without exciting theundesired mode.

For example, in the White et al. article en-tiled Additional Informationon A Interaction Circuit for Traveling Wave Tubes there is shown in FIG.1(d) an w-,8 diagram which illustrates the transverse (T) andlongitudinal (L) modes of the ring-plane circuit of FIG. 1(b). Theversion of the ring-plane circuit therein discussed comprises a pair ofconductive plates, positioned in a common plane, between which areattached a series of spaced conductive rings. In the longitudinal mode,the one ordinarily desired for traveling-wave tube interaction, oppositesides of the rings in any transverse plane of the ring plane circuit areexcited in phase whereby predominantly longitudinal electric fieldsexist between successive rings and on the axis. On the other hand, thetransverse mode is characterized by an out-of-phase excitation ofopposite sides of the rings which consequent strong transverse electricfields. Thus the ring-plane slow-wave circuit, for example, has at leasttwo modes of propagation "having nearly equal phase velocities whichmakes difficult the excitation of nly the desired mode.

3,292,112 Patented Dec. 13, 1966 It is therefore a primary object of theinvention to provide a coupling arrangement for coupling to a multimodecircuit electromagnetic energy in a selected mode.

It is a further object of the invention to terminate reflectionlesslythe undesired modes of a multi-mode circuit whereby reflection-inducedoscillations are inhibited.

These and other objects of the invention are achieved by providingseparate coupling channels to or from separated points of a suitablewave transmission circuit, such as the opposite sides of the planesymmetry of a ringplane circuit. These separate channels, which thusseparately transmit equal portions of electromagnetic energy, areconnected to the separate symmetrical ports of a suitablephase-sensitive four-branch microwave network, for example, a magic teecircuit, which directs energy in the different modes to differentbranches or ports, discrimination among the modes being based upon therelative phasing of the electromagnetic waves.

Thus in its use as an output coupler, the separated energy from the wavetransmission circuit is transmitted by the separate coupling channels tothe symmetrical ports of the magic tee or other phase-sensitive circuit.Because of the phase difference, the energy of the two modes isseparated by the magic tee circuit, the longitudinal or in-phase modeenergy appearing at the corresponding asymmetrical port of the magic tee(hereinafter called the H-plane port) while the transverse orout-of-phase mode energy appears at the other asymmetrical port of themagic tee (hereiafter called the E-plane port). The port providing thedesired mode (usually the -H-plane port) is connected to a utilizationcircuit while the port corresponding to the undesired mode is ordinarilyterminated in a matched load to dissipate the energy of the undesiredmode thereby preventing its reflection whereby reflection-inducedoscillation is avoided.

In its use as an input coupler, input energy is applied to theasymmetrical port corresponding to the desired mode, the otherasymmetrical port being terminated in a matched load. The input energydivides in the magic tee or phase-sensitive circuit. This divided energyappears at the symmetrical ports of the magic tee and is transmitted andapplied by the coupling channels to the two sides of the plane ofsymmetry of the wave transmission circuit or points corresponding toopposite phase relation.

Thus very effective separation and control of the multiple modes isachieved even though they are not widely separated in terms of phasevelocity.

The invention is described more specifically with reference to theaccompanying drawing in which:

FIGURE 1 is a schematic illustration showing the selective couplersystem of the invention as employed with a traveling wave tube;

FIGURE 2 is a perspective illustration of a preferred form of wavetransmission circuit;

FIGURE 3 is a perspective illustration of a selective coupler accordingto the invention; and

FIGURE 4 is a cut-away perspective illustration showing the couplingchannels and transition section and their relation to the wavetransmission circuit.

FIG. 1 schematically illustrates the use of selective coupiers accordingto the invention as both the input and output coupling means to the wavetransmission or slowwave circuit of a traveling wave tube.

A traveling wave tube is illustrated schematically in FIG. 1 ascomprising an electron gun, including a cathode 10 and a plurality offocusing and accelerating electrodes 11 and 12 for projecting a beam ofelectrons adjacent a Wave transmission or slow-wave circuit 13 to acollector electrode 14. These elements of the traveling-wave tube areenclosed in an evacuated envelope (not shown) and appropriate operatingvoltages are applied to terminals 15, 16, 17 and 18 connected torespective tube elements. Also it is ordinarily necessary to providemeans (not shown) for focusing the electron beam along its path betweenthe cathode and the collector electrode. Such focusing may beaccomplished, for example, by a magnet structure providing a magneticfield parallel to the path of the beam.

The wave transmission or slow-wave circuit 13 is characterized as beinga two-sided circuit having a longitudinal plane of symmetry and havingtwo modes of propagation within the frequency range of interest, alongitudinal mode wherein the two sides are excited in-phase and atransverse mode wherein the two sides are excited out-ofphase. Thecoupler of the invention provides the coupling of electromagnetic energyto the slow-Wave circuit in a selected mode.

As illustrated in FIG. 1 an input coupler 19 includes a pair of couplingchannels 20 and 21 and a four-branch network, illustrated as a magic teejunction 22. The well-known magic tee waveguide junction 22 includes apair of symmetrical ports 23 and 24 and a pair of asymmetrical ports 25and 26. (Such magic tee junctions are discussed, for example, byMontgomery et al. in a book entitled Principles of Microwave Circuits,Radiation Laboratory Series, pages 306 and 452, McGraw-I-Iill, New York,1948.)

The magic tee junction has the property of dividing (or combining)microwave energy on the basis of phase relationship and directing theenergy of different phase relation (corresponding to different modes) todifferent ports. In the present coupler structure the asymmetrical port25, designated the H-plane port, corresponds to the longitudinal orin-phase anode. The asymmetrical port 24, designated the E-plane port,corresponds to the transverse or out-of-phase mode.

Operation of the input coupler 19 is as follows: Input electromagneticenergy is applied from a source 42 to the appropriate asymmetrical portof the magic tee 22 depending upon the mode of excitation desired. Inthe present example it is assumed that the longitudinal mode is thedesired mode. Therefore the input energy is applied from source 42 tothe H-plane port 25. The magic tee 22 operates to divide the inputenergy equally and to thus direct equal portions thereof to thesymmertical ports 23 and 24 and thence into the coupling channels 20 and21. The coupling channels 20 and 21 include a respective pair oftransition sections 28 and 29 by which the divided input energy isapplied equally to opposite sides of the slow-wave circuit 13 forpropagation thereby. The E-plane port 26 is terminated by a matchedload, illustrated as a resistance 27, whereby out-of-phase energy whichmay appear at this port is absorbed and not reflected into the system.

The output coupler 19' is similar to the input coupler 19 and includessimilar elements as indicated by the primed reference numbers. Thetransition sections 28'. and 29' couple the electromagnetic energy fromrespective sides of the wave transmission circuit 13 into the couplingchannels 20' and 21'. The thus separated output energy is applied by thecoupling channels 20' and 21 to the symmetrical ports 23 and 24" of themagic tee junction 22'. (It is noted that the coupling channels areformed with equal waveguide lengths from the transition regions adjacentthe wave transmission circuit to the symmetrical ports of the magic teeso that the phase relation of divided output energy is maintained.)

Upon receipt of the divided output energy by symmetrical ports 23' and24', the magic tee junction 22 operates to combine the energy accordingto its phase relation by which the in-phase energy is directed to theH-plane port 25' and any out-of-phase (or transverse mode) energy isdirected to the E-plane port 26'.

The -H-plane port 25' is connected to a utilization circuit 43. TheE-plane port 26 is terminated in a matched load, represented as aresistor 27' in FIG. 1, by which reflection of the transverse modeenergy is substantially prevented to thereby prevent reflection-inducedoscillations in the system. The energy from the E-plane port may, ofcourse, be otherwise utilized if desired.

The wave transmission circuit 13 of FIG. 1 may advantageously comprise aring-plane slow-wave circuit 313 the references therein cited, it beingsufficient for present purposes to observe that this circuit canadvantageously be employed in a traveling wave tube for the desired highfrequency and high power operation and that it supports two modes asillustrated by the 01-5 diagram of FIG. 1(d) of the foregoing article.

FIG. 3, is a perspective illustration of the selective coupler of theinvention. As illustrated in FIG. 3, a select-ive coupler 319 comprisesa pair of coupling channels formed by waveguides 320 and 321, a magictee waveguide junction 322 having symmetrical ports 323 and 324 andasymmetrical ports 325 and 326, and a pair of transition sections 328and 329 for coupling the coupling waveguides 320 and 321 to a wavetransmission circuit 313.

Shown in FIG. 4 is a partly cut away view showing the interior of thecoupling waveguides 320 and 321 of FIG. 3 and illustrating in morespecific detail a preferred form of the transition sections 328 and 329.The transition section 329, for example, includes opposing surfaces 40and 41 which form a curved, tapering transition section by whichelectromagnetic energy is launched upon, or received from, therespective side of the wave transmission circuit 313.

Thus what has been described is a selective coupler structure forcoupling electromagnetic energy in a selected mode to a multimode wavetransmission circuit while suppressing excitation of undesired modeswhereby such multimode wave transmission circuits may be advantageouslyemployed in a high-frequency, high-power traveling-wave tube or thelike.

Although the invention has been described with reference to anillustrated embodiment of a multimode wave transmission circuit having asingle plane of symmetry, the principles of the invention as taughtherein may be extended and applied to wave transmission circuits havinga plurality of planes of symmetry such as shown, for example in FIG.1(0) of the previously mentioned White et al. article. While theprinciples of the invention have been made clear in the illustrativeembodiments, there will be obvious to those skilled in the art, manymodifications in structure, arrangement, proportions, the elements,materials, and components, used in the practice of the invention, andotherwise, which are adapted for specific environments and operatingrequirements, Without departing from these principles. The appendedclaims are therefore intended to cover and embrace any suchmodifications within the limits only of the true spirit and scope of theinvention.

What is claimed is:

1. The combination of: a two-sided slow wave transmission circuit havinga longitudinal plane of symmetry,

thereby providing two modes of propagation of electromagnetic energy, afirst mode characterized by being in phase with respect to the twotransverse sides of the plane of symmetry of said circuit and a secondmode characterized by being out-of-phase with respect to said two sides;a pair of coupling channels adapted to separately receiveelectromagnetic energy from respective sides of said circuit; and anelectromagnetic wave network having four branches for receiving energyfrom said coupling channels at first and second branches and fordifecting energy in said first mode to a third one of said branches andfor directing energy in said second mode to a fourth one of saidbranches.

2. The combination of: a two-sided slow wave transmission circuit havinga longitudinal plane of symmetry, thereby providing two modes ofpropagation of electromagnetic energy, a first mode characterized bybeing in phase with respect to the two transverse sides of the plane ofsymmetry of said circuit and a second mode characterized by beingout-of-phase with respect to said two sides; a four branchelectromagnetic wave network for receiving electromagnetic energy in apredetermined mode at a respective first branch and to direct saidenergy equally to second and third branches; and a pair of couplingchannels for coupling said energy from said second and third branchesseparately to respective sides of said wave transmission circuit.

3. The combination of: an elongated two-sided slow wave transmissioncircuit having a longitudinal plane of symmetry, thereby providingpropagation of electromagnetic Wave energy in transverse andlongitudinal modes; an electromagnetic wave network having first andsecond branches symmetrical to each other and having a third branchcorresponding to said transverse mode and a fourth branch correspondingto said longitudinal mode, said third and fourth branches beingasymmetrical to each other, said network directing to said symmetricalbranches equal portions of electromagnetic energy applied to a selectedone of said asymmetrical branches in a corresponding mode, and directingelectromagnetic energy received at said symmetrical branches to saidasymmetrical branches according to its mode; and a pair of couplingchannels forming separate electromagnetic energy coupling circuitsbetween said symmetrical branches of said network and said Wavetransmission circuit.

4. A circuit adapted for slow-wave transmission of electromagnetic waveenergy between a source and a utilization circuit, comprising: atwo-sided slow wave transmission circuit having a longitudinal plane ofsymmetry, thereby propagating electromagnetic wave energy in transverseand longitudinal modes; a mode-selective input coupling circuit forlaunching said wave energy from said source upon said wave transmissioncircuit; and a mode-selective output coupling circuit for coupling waveenergy from said wave transmission circuits to said utilization circuit,said output coupling circuit including a pair of coupling channelstransmitting equal portions of wave energy from separated points on saidwave transmission circuit, and a four-branch phase-sensitive network forreceiving said equal portions of wave energy from said coupling channelsat respective ones of two of said branches, for directing the waveenergy in said longitudinal mode to a third one of said branches and fordirecting the wave energy in said transverse mode to a fourth one ofsaid branches.

References Cited by the Examiner UNITED STATES PATENTS 2,851,681 9/1958Cohn 333-11 X 2,991,471 7/1961 Pritchard 333-11 X 3,058,071 10/1962Walsh et al 333-11 HERMAN KARL SAALBACH, Primary Examiner.

P. L. GENSLER, Assistant Examiner.

1. THE COMBINATION OF: A TWO-SIDED SLOW WAVE TRANSMISSION CIRCUIT HAVINGA LONGITUDINAL PLANE OF SYMMETRY, THEREBY PROVIDING TWO MODES OFPROPAGATION OF ELECTROMAGNETIC ENERGY, A FIRST MODE CHARACTERIZED BYBEING IN PHASE WITH RESPECT TO THE TWO TRANSVERSE SIDES OF THE PLANE OFSYMMETRY OF SAID CIRCUIT AND A SECOND MODE CHARACTERIZED BY BEINGOUT-OF-PHASE WITH RESPECT TO SAID TWO SIDES; A PAIR OF COUPLING CHANNELSADAPTED TO SEPARATELY RECEIVE ELECTROMAGNETIC ENERGY FROM RESPECTIVESIDES OF SAID CIRCUIT; AND AN ELECTROMAGNETIC WAVE NETWORK HAVING FOURBRANCHES FOR RECEIVING ENERGY FROM SAID COUPLING CHANNELS AT FIRST ANDSECOND BRANCHES AND FOR DIRECTING ENERGY IN SAID FIRST MODE TO A THIRDONE OF SAID BRANCHES AND FOR DIRECTING ENERGY IN SAID SECOND MODE TO AFOURTH ONE OF SAID BRANCHES.